The present invention relates to an image processing element; and, more particularly, to an image sensor having a plurality of unit pixels for preventing a blooming effect.
As is well known, an image sensor is an apparatus for sensing a light reflected from an object to generate an image data. Especially, an image sensor fabricated by using a complementary metal oxide semiconductor (CMOS) technology is called a CMOS image sensor.
Generally, the CMOS image sensor includes a plurality of unit pixels. Each of the unit pixels also includes a light sensing element and a plurality of transistors. The light sensing element such as a photodiode senses incident light reflected from an object and accumulates photoelectric charges that are generated due to the incident light. The transistors control a transfer of the photoelectric charges.
FIG. 1A is a circuit diagram showing a conventional unit pixel contained in a CMOS image sensor. A reference numeral 160 represents a load transistor used for stabilizing an output signal of the unit pixel. FIG. 1B is a diagram showing a layout of a conventional unit pixel shown in FIG. 1A.
As shown, the conventional unit pixel includes one photodiode 110 and four NMOS transistors. Four NMOS transistors also include a transfer transistor 120, a reset transistor 130, an amplification transistor 140, and a switching transistor 150.
The photodiode 110 senses a light beam incident thereto and generates photoelectric charges.
The transfer transistor 120, coupled to a sensing node Ns, transfers the photoelectric charges to the sensing node Ns in response to a transfer control signal TX. The reset transistor 130, coupled to the sensing node Ns, makes a fully depleted region within the photodiode 110 and provides a reset voltage to the sensing node in response to a reset control signal RX.
The amplification transistor 140 amplifies a voltage level of the sensing node Ns to generate an amplified signal. The switching transistor 150, coupled to the amplification transistor 140 and an output terminal Nout, performs a switching operation to output the amplified signal as an image data via the output terminal Nout in response to a switching control signal SX.
Among four NMOS transistors, the transfer transistor 120 and the reset transistor 130 are implemented with a depletion-mode NMOS transistor or a negative NMOS transistor having low threshold voltage in order to improve a charge transfer efficiency and reduce a voltage loss or a voltage drop of the image data.
In such a conventional unit pixel, since a procedure of resetting the sensing node Ns is achieved by the transfer transistor 120 and the reset transistor 130, excess charges in a saturation region should be flowed from the transfer and the reset transistors 120 and 130 to a power line VDD. Therefore, it is very important to control a voltage barrier of the transfer transistor 120 and the reset transistor 130. If the voltage barrier in either the transfer transistor 120 or the reset transistor 130 is not controlled correctly, the excess charges may undesirably flow to neighboring unit pixels so that an erroneous operation is caused. This phenomenon is called a blooming effect. As a result, due to the blooming effect, it is difficult to obtain an exact image data, thereby reducing an optical sensitivity of a CMOS image sensor.
It is, therefore, an object of the present invention to provide a CMOS image sensor having a plurality of unit pixels for preventing a blooming effect.
In accordance with an aspect of the present invention, there is provided an image sensor having a plurality of unit pixels, each unit pixel comprising: a light sensing means for sensing a light beam incident thereto and generating photoelectric charges; a transferring means for transferring the photoelectric charges to a sensing node; a first resetting means for making a fully depleted region within the light sensing means and resetting the sensing node by providing a power supply voltage to the sensing node; and a second resetting means for transferring excess charges generated in the light sensing means to a power line when the sensing node is reset.